1. Field of the Invention
The invention pertains to antenna array systems of the type in which a plurality of cooperating antenna elements provides means for selectively generating multiple radiation or reception patterns and more particularly relates to such antenna arrays that are particularly adaptable to use in airborne microwave radiometric systems of the search or mapping kind.
2. Description of the Prior Art
Because of inherent characteristics of antenna systems employed in the prior art, it has been difficult to provide an antenna array particularly adapted for continuous wide frequency band scanning or viewing of terrain or ocean areas for surveying or for surveillance purposes, such as for warning of the presence of dangerous ice or iceberg conditions. Mapping or surveillance passive radiometric systems require a wide operational band width. Signal amplitudes being small and object-identifying gradients also small, very low loss, microwave (especially millimeter wave length) systems are desired. Prior art proposals have not been fully successful, in that low loss and sufficient band width have not generally been attained. Where prior art antennas view a sufficiently wide sector, serious deterioration of the beam shape and width is observed, especially at the extremes of the sector scanned or viewed. Beam shape deterioration and wide variation in the location and amplitude of undesired side lobes have been present, in part due to lack of uniform energy phase fronts in various parts of the antenna systems.
A known prior approach to the problem is that described in the A. H. Schaufelberger U.S. Pat. No. 3,697,988 for a "Multiple Beam Array Antenna," issued Oct. 10, 1972 and assigned to Sperry Rand Corporation. This device is a multi-element antenna array system adapted to operation in either passive or active electronic systems and having an array antenna conforming to the cylindrical contour of an airborne vehicle. Elements of the array, such as slotted transmission line antennas in side-by-side cooperative relation, provide collimation in one plane of the radially extending antenna patterns. According to the prior invention, a plurality of such radially directive patterns may be simultaneously formed or one or more such patterns may be angularly scanned over a wide sector. The pattern generation mechanism employs a geodesic parallel plate energy guiding system which determines the activities of the antenna patterns and also additionally collimates them in a second plane.
However, the prior device by its inherent nature has proven to be unacceptably large for certain applications, requiring a volume not at all compatible with the small size of certain supersonic vehicles. In addition, the prior antenna device requires the use of a geodesic lens formed by a complex parallel plate conformal horn arrangement difficult and expensive to fabricate and to assemble. The simple flat parallel plate collimator of the present invention is much less expensive and less difficult to manufacture. Furthermore, the system aperture of the prior art device requires expansion to permit scanning, while the present invention makes efficient use of the available aperture and requires no additional aperture to accommodate scanning.